A drawn and ironed can has hitherto been formed as shown in FIG. 1. After a blank is punched from a metal sheet and drawn into a cup, an ironing apparatus having a punch 2 and a plurality of ironing dies 3 is used to finish the drawn cup into a can of predetermined sidewall thickness and height by setting the cup on the punch 2 and inserting it with the punch through the ironing dies. Large quantities of lubricant oil and cooling water are used to lubricate and cool the material during its drawing and ironing.
Attempts have recently been made to produce a can of still smaller sidewall thickness by ironing a can body formed from a resin-coated metal sheet by a conventional method consisting mainly of drawing, in order to realize environment preservation and a further reduction in mass of the can body. However, when a drawn and ironed can is formed from a resin-coated metal sheet by employing a traditional apparatus for forming a drawn and ironed can as shown in FIG. 1, an organic resin lf coating a metal sheet 1m is soft as compared with the metal sheet so that the organic resin near the open end of a can body is sticked out from the open end of the can body and cut, as shown in FIG. 1, by a very high pressure occurring between the tool and the material at the time of ironing to form thread-like cutting as shown at 1h (hereinafter called resin hair) . The resin hair is more likely to occur when ironing after drawing achieves a total ironing ratio of 15% or more. The resin hair occurring in a process for making drawn and ironed cans from resin-coated metal sheets continuously adheres to the punch or ironing die and thereby damages the coating resin on the surface of another drawn can that is going to be ironed. Thus, it is very difficult to form a drawn and ironed can from a resin-coated metal sheet by employing a traditional apparatus for forming a drawn and ironed can.
As a method of preventing resin hair from occurring when a resin-coated metal sheet is formed into a can body, there is disclosed a method in which a circular organic resin-coated metal sheet is held by an annular holding member and a drawing die, a drawing punch installed coaxially with the holding member and the drawing die and movably into and out of the holding member and the drawing die are moved relative to each other so as to engage with each other, and the circular metal sheet is formed into a drawn cup, in which at least one of the annular holding member and the drawing die is moved away from its pressure on the remaining-flange portion immediately before the ending of the drawing process to release the rear end of the flange portion to complete its drawing and thereby prevent any resin hair from occurring (see, for example, Patent Literature 1).
This method is aimed at preventing any resin hair from occurring when a drawn cup is formed, and it is possible to employ for drawing the annular holding member and drawing die which are movable to any coaxial position, but as the ironing step of the drawing and ironing process for attaining the object of the present invention is a method employing a punch passed through the inside diameter of an ironing die, and the inside diameter of the ironing die and the outside diameter of the punch are invariable during the process, it is impossible to release a high pressure produced between the tool and the material during the ironing of a resin-coated metal as stated before.
The following is information on prior art literature to which the present application pertains:
Patent Literature 1: JP-A-05-154570